參數(shù)資料
型號(hào)： TEA1085T
廠商： NXP SEMICONDUCTORS
元件分類： 通信及網(wǎng)絡(luò)
英文描述： Listening-in circuit for line-powered telephone sets
中文描述： SPECIALTY TELECOM CIRCUIT, PDSO24
封裝： PLASTIC, SO-24
文件頁數(shù)： 11/32頁
文件大?。?/td> 199K
代理商： TEA1085T
March 1992
11
Philips Semiconductors
Preliminary specification
Listening-in circuit for line-powered
telephone sets
TEA1085; TEA1085A
Larsen limiter detector (DTI and DCA) pins 15 and 16
The QLA output signal is AC coupled to the detector input
DTI. DTI is biased by potential divider R30 and R31. The
voltage applied to DTI of the Larsen level limiter is
converted into a current for further processing in this
circuit. Current adjustment is achieved using the network
connected between DCA and V
BB
(see Fig.8).
The equation for DC current is:
R30
R31
+
The equation for AC current is:
V
R33
In the basic application:
R30 = 100 k
, R31 = 220 k
, R33 = 500
, R32 = 100 k
and C25 = 330 nF
This results in I
DCA
= 11
μ
A and the equation:
i
V
DTI
High-pass filter
A third order high-pass filter is created between the
microphone input voltage and the current flowing into
DCA. The cut-off frequencies (see Fig.9) of the three
sections are:
2
π
R
eg
C24
2
π
R33C24
Where: R25 = R26 and C22 = C23
The filter reduces the sensitivity of the system to own
speech.
Normal speech is in the frequency range 300 Hz to
3400 Hz, however, the Larsen signal normally occurs at a
frequency
>
3 kHz.
With the component values as used in the basic
application (see Fig.16); f1 = 500 Hz, f2 = 1 kHz and
f3 = 3 kHz
I
DCA
----------------------------
V
BB
+
R32
R33
----------------------------
×
×
=
i
DCA
-----------
for f >1
2
--
π
R33 C25
=
-----------
2 (mA/V)
=
f1
----------------------------
where R
eq
R30
R31
+
----------------------------
=
=
f2
------------------------------
=
f3
2
π
R26C23
------------------------------
1/(2
π
R25C22
)
=
=
Where:
Larsen limiter capacitor (LLC) pin 12
A 1
μ
F capacitor (C26) is connected externally between
V
SS
and LLC to determine the attack and release timing of
the Larsen level limiter in the listen-in and Larsen mode.
The timing is also dependent on the value of the resistor
connected between SIC and V
SS
.
Larsen level limiter threshold (THL1 and THL2) pins
13 and 14
When the signal at DTI exceeds the first threshold level the
capacitor connected to LLC will start to discharge. The first
threshold level is determined by the value of the resistor,
R35, connected to THL1 and V
SS
. The amount of
discharge of C26 depends on how much the level of the
signal at DTI exceeds the first threshold level (for normal
speech the discharge is small).
The Larsen effect is generally defined as a signal level of
≥
100 mV(RMS), on line, for a period of more than 100 ms.
The Larsen signal must be reduced to a low level within
200 ms. For Larsen signal levels (f
>
f3 in Fig.9) of
≥
100 mV(RMS) at DTI and, with the component values of
Fig.16, the system will switch from the listen-in mode to the
Larsen mode in a time period of 100 ms to 200 ms;
consequently, the initial Larsen effect will last only for a
short period of time.
Fig.9 Third-order high-pass filter.
handbook, halfpage
20 log
MGR038
(dB)
go
20 log f
f3
f2
f1
0
12 dB per octave
6 dB per octave
18 dB per octave
speech
Larsen
g
i
V
m
-----------
=
g
o
A
----------
=
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